1. Field of the Invention
The present invention relates to a method for preventing corrosion of a bonding pad on a surface of a semiconductor wafer, and more particularly, to a method for preventing corrosion of a bonding pad resulting from residual polymers on a surface of a semiconductor wafer.
2. Description of the Prior Art
In a semiconductor process, when all devices and metallic connections are formed on a semiconductor wafer, insulating layers are deposited or coated on a surface of the wafer to passivate their underlying integrated circuits, and a plurality of holes is then etched into the insulating layers so as to expose bonding pads formed on the surface of the wafer. The bonding pads are used to connect external devices through a wire bonding process to enable transmissions of electric signals from the wafer to external devices. Generally, the insulating layers and the plurality of holes are formed in an integrated circuit (IC) foundry, and the wafer is transported to an IC packaging factory for performing the wire bonding process. Because the wire bonding process is performed several days after forming the insulating layers and the holes, the bonding pads are exposed to open air for a long time causing a chemical reaction to occur. This chemical reaction will damage the bonding pads thus making the entire wafer abandoned.
Please refer to FIG. l. FIG. 1 is a flowchart of a prior art process 10 of forming insulating layers and holes on bonding pads of a wafer. The process 10 is performed in an IC foundry in five steps when patterns are transferred onto the wafer: First, deposit a passivation layer on the wafer; second, coat a polyimide layer on the passivation layer; third, remove polyimide above the bonding pads through a photolithographic process; fourth, cure remaining polyimide on the wafer; fifth, perform a dry etching process in areas where the polyimide is removed to expose the bonding pads. When the process 10 is completed, the wafer is transported from the IC foundry to the IC packaging factory to perform the wire bonding process.
The dry etching process in the fifth step is performed by using a mixed gas comprising carbide, sulphide and fluoride, and commonly causes the mixed gas to generate polymers. The polymers will remain on the bonding pads when the dry etching process is completed, and will react with water vapors to generate acidic chemicals. Because the polyimide is organic photo-resist, using a solvent to clean off the polymers is not possible. Moreover, the acidic chemicals are sources of corrosion of the bonding pads. Thus, the bonding pads will become damaged and the entire wafer will be abandoned.